There is a need in a variety of situations to drill, intersect and connect two boreholes together where the intersection and connection is done below ground. For instance, it may be desirable to achieve intersection between boreholes when drilling relief boreholes, drilling underground passages such as river crossings, or when linking a new borehole with a producing wellbore. A pair of such intersected and connecting boreholes may be referred to as a “U-tube borehole”.
For example, Steam Assisted Gravity Drainage (“SAGD”) may be employed in two connected or intersecting boreholes, in which the steam is injected at one end of the U-tube borehole and production occurs at the other end of the U-tube borehole. More particularly, the injection of steam into one end of the U-tube borehole reduces the viscosity of hydrocarbons which are contained in the formations adjacent to the borehole and enables the hydrocarbons to flow toward the borehole. The hydrocarbons may then be produced from the other end of the U-tube borehole using conventional production techniques. Specific examples are described in U.S. Pat. No. 5,655,605 issued Aug. 12, 1997 to Matthews and U.S. Pat. No. 6,263,965 issued Jul. 24, 2001 to Schmidt et. al.
Other potential applications or benefits of the creation of a U-tube borehole include the creation of underground pipelines to carry fluids, which include liquids and/or gases, from one location to another where traversing the surface or the sea floor with an above ground or conventional pipeline presents a relatively high cost or a potentially unacceptable impact on the environment.
Such situations may exist where the pipeline is required to traverse deep gorges on land or on the sea floor. Further, such situations may exist where the pipeline is required to traverse a shoreline with high cliffs or sensitive coastal marine areas that can not be disturbed. In addition, going across bodies of water such as lake beds, river basins or harbors may be detrimental to the environment in the event of breakage of an above ground or conventional pipeline. In sensitive areas, conventional above ground pipelines would simply not be acceptable because of the environmental risk. Further, locating the pipeline below the lake bed or sea floor provides an extra level of security against leakage.
River crossing drilling rigs are presently utilized to perform such drilling on a routine basis around the world. Conventional river crossing drilling requires that the borehole enter at one surface location and drill back to surface at the second location. Since most of these holes are relatively short there is less concern about drag and the effects of gravity as the drilling rig typically has ample push to achieve the goal over such a short interval. However, concerns regarding drag and the effects of gravity increase with the length of the borehole.
Further, conventional river crossing drilling rigs tend to have a limited reach. In some instances, there is simply not enough lateral reach to drill down and then exit back up at the surface on the other side of the obstacle that is trying to be avoided. Also, in the event that the borehole enters into a pressurized formation, exiting on the other side at the surface presents safety issues as no well control measures, such as a blow-out preventer (“BOP”) and cemented casing, are present at the exit point.
Thus, one clear benefit of using two surface locations instead of one is that the effective distance possible between the two locations can be at least doubled as torque and drag limitations can be maximized for reach at both surface locations. Further, necessary well control and safety measures may be provided at each surface location.
Further, in some areas of the world, such as offshore of the east coast of Canada, icebergs have rendered seabed pipelines impractical in some places since the iceberg can gouge long trenches in the sea floor as it floats by, thus tearing up the pipeline. This essentially means that a gravity based structure, such as that utilized in Hibernia, must be utilized to protect the well and the interconnecting pipe from being hit by the iceberg at a massive cost.
Therefore, there is a need for a method for drilling relatively long underground pipelines by drilling from two separate or spaced apart surface locations and then intersecting the boreholes at a location beneath the surface in order to connect the two surface locations together.
In order to permit the drilling of a U-tube borehole or underground pipeline, careful control must be maintained during the drilling of the boreholes, preferably with respect to both the orientation of the intersecting borehole relative to the target borehole and the separation distance between the intersecting and target boreholes, in order to achieve the desired intersection. This control can be achieved using magnetic ranging techniques.
Magnetic ranging is a general term which is used to describe a variety of techniques which use magnetic field measurements to determine the relative position (i.e., relative orientation and/or separation distance) of a borehole being drilled relative to a target such as another borehole or boreholes.
Magnetic ranging techniques include both “passive” techniques and “active” techniques. In both cases, the position of a borehole being drilled is compared with the position of a target such as a target borehole or some other reference such as ground surface. A discussion of both passive magnetic ranging techniques and active magnetic ranging techniques may be found in Grills, Tracy, “Magnetic Ranging Techniques for Drilling Steam Assisted Gravity Drainage Well Pairs and Unique Well Geometries—A Comparison of Technologies”, SPE/Petroleum Society of CIM/CHOA 79005, 2002.
Passive magnetic ranging techniques, sometimes referred to as magnetostatic techniques, typically involve the measurement of residual or remnant magnetism in a target borehole using a measurement device or devices which are placed in a borehole being drilled.
An advantage of passive magnetic ranging techniques is that they do not typically require access into the target borehole since the magnetic field measurements are taken of the target borehole “as is”. One disadvantage of passive magnetic ranging techniques is that they do require relatively accurate knowledge of the local magnitude and direction of the earth's magnetic field, since the magnetic field measurements which are taken represent a combination of the magnetism inherent in the target borehole and the local values of the earth's magnetic field. A second disadvantage of passive magnetic ranging techniques is that they do not provide for control over the magnetic fields which give rise to the magnetic field measurements.
Active magnetic ranging techniques commonly involve the measurement, in one of a target borehole or a borehole being drilled, of one or more magnetic fields which are created in the other of the target borehole or the borehole being drilled.
A disadvantage of active magnetic ranging techniques is that they do typically require access into the target borehole in order either to create the magnetic field or fields or to make the magnetic field measurements. One advantage of active magnetic ranging techniques is that they offer full control over the magnetic field or fields being created. Specifically, the magnitude and geometry of the magnetic field or fields can be controlled, and varying magnetic fields of desired frequencies can be created. A second advantage of active magnetic ranging techniques is that they do not typically require accurate knowledge of the local magnitude and direction of the earth's magnetic field because the influence of the earth's magnetic field can be cancelled or eliminated from the measurements of the created magnetic field or fields.
As a result, active magnetic ranging techniques are generally preferred where access into the target borehole is possible, since active magnetic ranging techniques have been found to be relatively reliable, robust and accurate.
One active magnetic ranging technique involves the use of a varying magnetic field source. The varying magnetic field source may be comprised of an electromagnet such as a solenoid which is driven by a varying electrical signal such as an alternating current in order to produce a varying magnetic field. Alternatively, the varying magnetic field source may be comprised of a magnet which is rotated in order to generate a varying magnetic field.
In either case, the specific characteristics of the varying magnetic field enable the magnetic field to be distinguished from other magnetic influences which may be present due to residual magnetism in the borehole or due to the earth's magnetic field. In addition, the use of an alternating magnetic field in which the polarity of the magnetic field changes periodically facilitates the cancellation or elimination from measurements of constant magnetic field influences such as residual magnetism in ferromagnetic components, such as tubing, casing or liner, positioned in the borehole or the earth's magnetic field.
The varying magnetic field may be generated in the target borehole, in which case the varying magnetic field is measured in the borehole being drilled. Alternatively, the varying magnetic field may be generated in the borehole being drilled, in which case the varying magnetic field is measured in the target borehole.
The varying magnetic field may be configured so that the “axis” of the magnetic field is in any orientation relative to the borehole. Typically, the varying magnetic field is configured so that the axis of the magnetic field is oriented either parallel to the borehole or perpendicular to the borehole.
U.S. Pat. No. 4,621,698 (Pittard et al) describes a percussion boring tool which includes a pair of coils mounted at the back end thereof. One of the coils produces a magnetic field parallel to the axis of the tool and the other of the coils produces a magnetic field transverse to the axis of the tool. The coils are intermittently excited by a low frequency generator. Two crossed sensor coils are positioned remote of the tool such that a line perpendicular to the axes of the sensor coils defines a boresite axis. The position of the tool relative to the boresite axis is determined using magnetic field measurements obtained from the sensor coils of the magnetic fields produced by the coils mounted in the tool.
U.S. Pat. No. 5,002,137 (Dickinson et al) describes a percussive action mole including a mole head having a slant face, behind which slant face is mounted a transverse permanent magnet or an electromagnet. Rotation of the mole results in the generation of a varying magnetic field by the magnet, which varying magnetic field is measured at the ground surface by an arrangement of magnetometers in order to obtain magnetic field measurements which are used to determine the position of the mole relative to the magnetometers.
U.S. Pat. No. 5,258,755 (Kuckes) describes a magnetic field guidance system for guiding a movable carrier such as a drill assembly with respect to a fixed target such as a target borehole. The system includes two varying magnetic field sources which are mounted within a drill collar in the drilling assembly so that the varying magnetic field sources can be inserted in a borehole being drilled. One of the varying magnetic field sources is a solenoid axially aligned with the drill collar which generates a varying magnetic field by being driven by an alternating electrical current. The other of the varying magnetic field sources is a permanent magnet which is mounted so as to be perpendicular to the axis of the drill collar and which rotates with the drill assembly to provide a varying magnetic field. The system further includes a three component fluxgate magnetometer which may be inserted in a target borehole in order to make magnetic field measurements of the varying magnetic fields generated by the varying magnetic field sources. The position of the borehole being drilled relative to the target is determined by processing the magnetic field measurements derived from the two varying magnetic field sources.
U.S. Pat. No. 5,589,775 (Kuckes) describes a method for determining the distance and direction from a first borehole to a second borehole which includes generating, by way of a rotating magnetic field source at a first location in the second borehole, an elliptically polarized magnetic field in the region of the first borehole. The method further includes positioning sensors at an observation point in the first borehole in order to make magnetic field measurements of the varying magnetic field generated by the rotating magnetic field source. The magnetic field source is a permanent magnet which is mounted in a non-magnetic piece of drill pipe which is located in a drill assembly just behind the drill bit. The magnet is mounted in the drill pipe so that the north-south axis of the magnet is perpendicular to the axis of rotation of the drill bit. The distance and direction from the first borehole to the second borehole are determined by processing the magnetic field measurements derived from the rotating magnetic field source.
Thus, there remains a need in the industry for a drilling method for connecting together at least two boreholes to provide or form at least one U-tube borehole. Further, there is a need for methods for completion of the U-tube borehole and methods for transferring material through the U-tube borehole or production of the U-tube borehole. Finally, there is a need for methods and for well configurations for interconnecting a plurality of the U-tube boreholes, preferably primarily below ground, to provide a network of U-tube boreholes capable of being produced or transferring material therethrough.